An essential function of lubricating oils for internal combustion engines is lubrication of various sliding portions such as piston rings, cylinder liners, crankshafts, and connecting rods, and lubrication of sliding portions of a valve-operating mechanism including a cam and a valve-lifter. In addition, lubricating oils are able to cool the inside of such engines and disperse sludge and uncombusted fuel.
Thus, lubricating oils for internal combustion engines are required to exhibit various properties, and requirements for such properties are becoming more and more severe in accordance with recent years' trends toward lower fuel consumption, higher power, higher driving condition adaptability, etc. of internal combustion engines.
In particularly, exhaust gas cleaning apparatuses (inter alia, particulate filter or an exhaust gas cleaning apparatus) have been developed for diesel engines, and lubricating oil compositions are known to affect such exhaust gas cleaning apparatuses.
A lubricating oil composition containing a metallic detergent may cause deposition, in a filter, of a metallic component originating from the additive, resulting in plugging of the filter and lowering of catalytic activity. Therefore, demand has arisen for lowering the ash content of lubricating oil.
When a metallic detergent and an anti-wear agent, which are additives containing a metallic component, are used in reduced amounts, antioxidation performance and heat resistance are impaired. In this case, impairment of such a lubricating oil is accelerated, to thereby shorten the interval (distance or time) required for changing lubricating oil.
Meanwhile, in order to attain complete combustion of light oil fuel, the dead volume above the piston must be reduced. Therefore, recently, pistons with a higher top ring placement as compared with conventional pistons have become popular.
As the top ring position is elevated, the employed lubricating oil is subjected to higher temperature conditions, and viscosity and acid number of the lubricating oil increase due to oxidation-related impairment, thereby shortening the interval (distance or time) required for changing lubricating oil.
Addition of a phenol compound or an amine compound to a lubricating oil composition is a conventional technology for enhancing oxidation stability of the composition (see, for example, Patent Documents 1 to 11).
Another conventional technology for preventing impairment of a lubricating oil is addition, to a lube base oil, of an organic zinc dithiophosphate having secondary hydrocarbon group having 5 to 20 carbon atoms, an alkaline earth metal salicylate salt, and a boron-containing derivative of an alkenylsuccinimide having a boron/nitrogen atomic ratio of 0.2 to 0.4 (see, for example, Patent Document 12). However, oxidation stability of the lubricating oil is not satisfactorily enhanced through this technique.
In a technique for enhancing oxidation stability against NOX, (a) polyalkenylsuccinimide and/or a boron-containing derivative thereof (4 to 10% by wt.), (b) a specific diarylamine (0.7 to 1.5% by wt.), and (C) a specific hindered phenol (0.7 to 1.5% by wt.) are incorporated into a lube base oil (see, for example, Patent Document 13). In the Example section of the disclosed document, phenylnaphthylamine is employed as diarylamine. Enhancement in oxidation stability is unsatisfactory.
Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 6-93281
Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 7-331270
Patent Document 3: Japanese Patent Application Laid-Open (kokai) No. 8-302378
Patent Document 4: Japanese Patent Application Laid-Open (kokai) No. 9-3463
Patent Document 5: Japanese Patent Application Laid-Open (kokai) No. 9-71795
Patent Document 6: Japanese Patent Application Laid-Open (kokai) No. 2001-158896
Patent Document 7: Japanese Patent Application Laid-Open (kokai) No. 2002-53888
Patent Document 8: Japanese Patent Application Laid-Open (kokai) No. 2002-371292
Patent Document 9: Japanese Patent Application Laid-Open (kokai) No. 2003-327987
Patent Document 10: Japanese Patent Application Laid-Open (kokai) No. 2004-107556
Patent Document 11: Japanese Patent Application Laid-Open (kokai) No. 2004-131742
Patent Document 12: Japanese Patent Application Laid-Open (kokai) No. 9-235579
Patent Document 13: Japanese Patent Application Laid-Open (kokai) No. 7-126681